Receiving a linearly polarized microwave signal from a ground station by a moving vehicle is a problem which has existed for some time. A typical solution to this problem is to mount an antenna known to provide omni-directional radiation on the vehicle, such as a crossed dipole, or the like. However, for certain kinds of vehicles, such as aircraft, the construction and mounting of such a typical omni-directional antenna can be cumbersome, inconvenient, or even not feasible due to the excessive speeds and limited mounting space available within the aircraft. Of course, the reason for using an omni-directional antenna is to ensure reception of the linearly polarized signal from a ground station or the like as the orientation of the aircraft changes with respect to the ground station.
To solve these and other problems in the prior art, the inventors herein have succeeded in developing a substantially omni-directional radiating antenna suitable for reception of a linearly polarized signal from a ground station but which uses microstrip patch radiators for the active elements. As is known in the art, microstrip patch radiators are relatively easy to build, such as by chemical etching of conductors on copper clad circuit board modules comprised of one or more layers, and are thin, flat, and conformable. These radiators are particularly well suited for use in an aircraft as they may be readily shaped to fit the curvature of the outer surface. In the first embodiment of the present invention, a pair of microstrip patch radiators are mounted back-to-back, each radiator being mounted on a metal plate which serves both as its mounting shelf and ground plane, the ground planes being mounted in close proximity but spaced apart from each other such that feed conductors may be routed through the space between the ground planes to feed each of the radiators without interfering with their radiation pattern. The radiators are fed to produce circular polarization of opposite senses. This results in constructive field addition along the antenna's ground plane.
In a second embodiment, the entire isopatch antenna is fabricated as a single unit with only one feed port. This eliminates the need for separate microstrip elements to be mounted on separate ground planes by utilizing a completely internalized feed network comprised of a power divider and combiner. This feed network is sandwiched between a pair of dielectric substrate panels which in turn are bonded between the two ground planes of the microstrip patch radiators. A single RF input connector is mounted to the dielectric substrate material with a strip line transmission line connecting it to the internal feed network which then separates the incoming signal as appropriate to feed the two microstrip patch radiators in accordance with the teachings of the invention.
As is known in the art, the patch on each radiator may be fed by a pair of feed points, the feed points being 90.degree. (physical) apart with the feed points of one radiator being 180.degree. (physical) from the feed points of the other radiator. These feed points may then be fed by a simple feed circuit which splits the signal into separate components of half power at both 0.degree. and 90.degree. phase delay. Alternately, the physical location of the feed points and the electrical delays may be changed to suit the particular application, as is well known in the art.
In the particular application disclosed herein, the isopatch antenna is particularly suitable for use in a data link communicating between the aircraft and a ground instrumentation station. The antenna is placed within a radome on a captive carry pod on the aircraft and, as is expected, the aircraft and pod could take virtually any orientation to the linearly polarized ground receiver station and yet receive the data because of the omni-directional radiation pattern of the antenna. The wide radiation pattern and circular polarization excitation utilized provides almost complete coverage between the antenna pod and ground station, the only hole in coverage occurring as the ground station would appear in the ground plane of the antenna such that a cross polarization would result. In other words, with the omni-directional antenna of the present invention, the radiation pattern reduces to linear polarization in the common ground plane such that if the linearly polarized signal from the ground station became oriented parallel to the ground plane, the signal and radiation pattern would be perpendicular or cross polarized, thereby greatly diminishing the power of the received signal. However, the chance of this particular orientation occurring can be minimized by thoughtful placement of the antenna within the air-craft pod.